Sebastian Vettel and Charles Leclerc have been among the fastest drivers in the first week of testing in Spain, with the Mercedes of Lewis Hamilton and Valtteri Bottas unable to keep pace.

Daniil Kvyat, back in F1 after a one-year absence, was the fastest in his Toro Rosso on the third of four test days at the Barcelona-Catalunya Circuit on Wednesday, ahead of former Ferrari driver Kimi Raikkonen, now with Alfa Romeo. Daniel Ricciardo was third with Renault, ahead of Vettel.

"Ferrari are very, very strong right now," said Hamilton, who was 12th fastest, just behind Bottas. "As you have seen, they are racking up great mileage as well and it appears that they have a better package than they had last year, which means it is going to be even more of a challenge for us this year."

Hamilton won his fifth world title last year, beating Vettel and Raikkonen in the drivers' championship.

"Ferrari always looks particularly strong, for the last few years they have looked very strong, so that is to be expected," Hamilton said. "For us, we are just going about our process and trying to understand, so everyone is working incredibly hard."

He said it's too early to be overly concerned with the rival's performance.

"It's not a time when we need to focus on others, we let everyone else do their thing and really dig deep and focus on making sure our processes are better than ever before, that we are analyzing the data better than ever before and that from a driver's point of view the feedback is more accurate than ever before," Hamilton said. "That is what we are trying to focus on and making sure we get through our run plans."

There was some concern that Mercedes may lose ground this year because it didn't do as well as Ferrari with the design of the new front wings, which are part of regulation changes for this upcoming season. Red Bull, which also is using a similar front-wing design to Mercedes, had Max Verstappen fifth-quickest after 109 laps on Wednesday.

Kvyat's time of 1 minute, 17.704 seconds was the fastest of the week. Raikkonen ran the most laps with 138, one more than Kvyat, and four more than Vettel.

Williams finally made it to the track after missing the first two days of testing because of delays with its 2019 car. Rookie George Russell managed only 23 laps and was the slowest driver of the day.

"It's embarrassing not bringing a race car to a circuit when everyone else has managed to do that, particularly a team like ours that has managed to bring a race car to testing for the past 40-odd years," Williams team principal Claire Williams said. "We can only apologize."

There were three red flags on Wednesday, all prompted by problems with Haas cars.

Testing will continue through Thursday and then again from Feb. 26-March 1 at the Barcelona-Catalunya Circuit, home to the Spanish Grand Prix in May.